Added tests for the respective AddAssign/Add operations for char concatenation to String and Cow<str>.

Author: Ruster-a11y

src/liballoc/tests/cow_str.rs

@@ -139,3 +139,33 @@ fn check_cow_clone_from() {
     c1.clone_from(&c2);
     assert!(c1.into_owned().capacity() >= 25);
 }
+
+#[test]
+fn check_cow_add_assign_char() {
+    let test_char = '👋';
+
+    let mut borrowed = Cow::Borrowed("Hello, World! ");
+    let borrow_empty = Cow::Borrowed("");
+
+    let mut owned: Cow<'_, str> = Cow::Owned(String::from("Hi, World! "));
+    let owned_empty: Cow<'_, str> = Cow::Owned(String::new());
+
+    let mut s = borrow_empty.clone();
+    s += test_char;
+    assert_eq!(test_char.to_string(), s);
+    if let Cow::Owned(_) = s {
+        panic!("Adding empty strings to a borrow should note allocate");
+    }
+    let mut s = owned_empty.clone();
+    s += test_char;
+    assert_eq!(test_char.to_string(), s);
+    if let Cow::Owned(_) = s {
+        panic!("Adding empty strings to a borrow should note allocate");
+    }
+
+    owned += test_char;
+    borrowed += test_char;
+
+    assert_eq!(format!("Hi, World! {}", test_char), owned);
+    assert_eq!(format!("Hello, World! {}", test_char), borrowed);
+}

src/liballoc/tests/string.rs

@@ -195,8 +195,10 @@ fn test_add_assign() {
     assert_eq!(s.as_str(), "");
     s += "abc";
     assert_eq!(s.as_str(), "abc");
-    s += "ประเทศไทย中华Việt Nam";
-    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam");
+    s += "ประเทศไทย中华Việt Nam ";
+    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam ");
+    s += '👋';
+    assert_eq!(s.as_str(), "abcประเทศไทย中华Việt Nam 👋")
 }
 
 #[test]
@@ -304,9 +306,10 @@ fn test_str_clear() {
 fn test_str_add() {
     let a = String::from("12345");
     let b = a + "2";
-    let b = b + "2";
-    assert_eq!(b.len(), 7);
-    assert_eq!(b, "1234522");
+    let b = b + "2 ";
+    let b = b + '👋';
+    assert_eq!(b.len(), 12);
+    assert_eq!(b, "1234522 👋");
 }
 
 #[test]